Bibcode
Villata, M.; Raiteri, C. M.; Kurtanidze, O. M.; Nikolashvili, M. G.; Ibrahimov, M. A.; Papadakis, I. E.; Tsinganos, K.; Sadakane, K.; Okada, N.; Takalo, L. O.; Sillanpää, A.; Tosti, G.; Ciprini, S.; Frasca, A.; Marilli, E.; Robb, R. M.; Noble, J. C.; Jorstad, S. G.; Hagen-Thorn, V. A.; Larionov, V. M.; Nesci, R.; Maesano, M.; Schwartz, R. D.; Basler, J.; Gorham, P. W.; Iwamatsu, H.; Kato, T.; Pullen, C.; Benítez, E.; de Diego, J. A.; Moilanen, M.; Oksanen, A.; Rodriguez, D.; Sadun, A. C.; Kelly, M.; Carini, M. T.; Miller, H. R.; Catalano, S.; Dultzin-Hacyan, D.; Fan, J. H.; Ishioka, R.; Karttunen, H.; Keinänen, P.; Kudryavtseva, N. A.; Lainela, M.; Lanteri, L.; Larionova, E. G.; Matsumoto, K.; Mattox, J. R.; Montagni, F.; Nucciarelli, G.; Ostorero, L.; Papamastorakis, J.; Pasanen, M.; Sobrito, G.; Uemura, M.
Referencia bibliográfica
Astronomy and Astrophysics, v.390, p.407-421 (2002)
Advertised on:
8
2002
Revista
Número de citas
155
Número de citas referidas
141
Descripción
We present UBVRI light curves of BL Lacertae from May
2000 to January 2001, obtained by 24 telescopes in 11 countries. More
than 15 000 observations were performed in that period, which was the
extension of the Whole Earth Blazar Telescope (WEBT) campaign originally
planned for July-August 2000. The exceptional sampling reached allows
one to follow the flux behaviour in fine detail. Two different phases
can be distinguished in the light curves: a first, relatively
low-brightness phase is followed by an outburst phase, after a more than
1 mag = brightening in a few weeks. Both the time duration (about 100 d)
and the variation amplitude (roughly 0.9 mag) are similar in the two
phases. Rapid flux oscillations are present all the time, involving
variations up to a few tenths of mag on hour time scales, and witnessing
an intense intraday activity of this source. In particular, a half-mag
brightness decrease in about 7 h was detected on August 8-9, 2000,
immediately followed by a ~ 0.4 mag brightening in 1.7 h. Colour indexes
have been derived by coupling the highest precision B and R data taken
by the same instrument within 20 min and after subtracting the host
galaxy contribution from the fluxes. The 620 indexes obtained show that
the optical spectrum is weakly sensitive to the long-term trend, while
it strictly follows the short-term flux behaviour, becoming bluer when
the brightness increases. Thus, spectral changes are not related to the
host galaxy contribution, but they are an intrinsic feature of fast
flares. We suggest that the achromatic mechanism causing the long-term
flux base-level modulation can be envisaged in a variation of the
relativistic Doppler beaming factor, and that this variation is likely
due to a change of the viewing angle. Discrete correlation function
(DCF) analysis reveals the existence of a characteristic time scale of
variability of ~ 7 h in the light curve of the core WEBT campaign, while
no measurable time delay between variations in the B and R bands is
found.